How to use a mesh to reveal an object
This is fairly straight-forward, there are two things you need to do:


Make the cube subtract from the sphere, and
Make the cube invisible to the camera so it doesn't render


First, select the sphere and add a Boolean Modifier to it, setting it to difference and making sure the "Object" option is set to the cube.



Now go ahead and set the maximum draw type for the cube to Wire in the object settings.



You should get a result like this:



Scroll down a little farther and under Cycles Settings un-check all the check-boxes. 



When you render you should have a result like this: